It seems that Nick Clegg is still adapting to government. Describing the Iraq war as “illegal”, as he did in the House of Commons yesterday, may have made sense as part of his election campaign messaging, but not something to be said by someone deputising for the prime minister at the despatch box unless it is the official government view.

Given that many of his cabinet colleagues voted for that war – one of them, work and pensions secretary Iain Duncan Smith was leader of the opposition at the time and a committed supporter of the war – this is unlikely to be the case.

It also seems that he is still adapting to coalition politics. He demanded that Jack Straw, for Labour, account for his support for the “illegal invasion of Iraq”, but less than three months previously, he had been considering Labour as a potential governing partner. Was this demand of the Labour party part of his negotiating position at the time?

In an era of coalition politics, today’s opponents might be tomorrow’s partners, and it is unwise to erect barriers too high in front of them. It is not only between the governing parties that coalition requires a change in the style of politics.

But my real concern about the use of the word “illegal” by a member of the government is not one of politics but of substance. What does it mean to declare that a war is, or was, illegal?

It puts me in mind of those European bishops who used to bless their respective national armies as they went off to war, telling them that they had God on their side, even though those armies were going to off to fight each other. The notion of the legality of a war is currently an incantation designed to make people feel better about what they are about to do. In the absence of a legal system, the notion of legality is essentially meaningless.

The proceedings of the Chilcot enquiry have already seen Jack Straw standing by his view that the Iraq war was legal and further, in a note of 29 January 2003 to his Legal Adviser, he declared that “There is no international court for resolving such questions in the manner of a domestic court.” This is precisely the problem.

Unlike God, though, international law has the virtue of being an entirely human creation and therefore can be changed and amended as needed. Creating the kind of court whose absence Jack Straw noted is perfectly possible.

If there is any advantage in a statement from a member of the government, it is that the government has the power to make things happen. It can propose laws, and change policies, and establish precedents on behalf of the country. One such action it should champion now is the creation of a court, or the endowment of the relevant on powers on one of the existing international courts, to rule on the legality of war. This would not arise as the result of a legal action between states, but from an action against the government of a state by its own citizens.

If anything could truly be described as the new politics, it is this. It would turn Nick Clegg’s pious platitudes about the war into a solid achievement.

Personally, I do not believe that Nick Clegg went into politics intending to be satisfied by pious platitudes alone, but I accept that he may yet prove me wrong.